A compressed air energy storage power plant consists of an underground storage space for the air, including an access well, and the surface power plant with a motor compressor and turbine-generator unit. There are basically two types of compressed air energy storage: the diabatic compressed air energy storage (Figure 1) disposes of and therefore wastes the heat generated when the air is compressed. This means that the air flowing out of the storage again during peak load periods has to be re-heated using natural gas to prevent the turbines from icing up. In the case of adiabatic compressed air energy storage (Figure 2), the heat generated by compression is held in a separate storage where it can be re-used to heat up the air again when it is withdrawn from the cavern. This dispenses with the use of natural gas and this therefore makes this type of CAES a pure power-to-power storage. The efficiency of diabatically-operated compressed air energy storage power plants is around 42 % to 54 %, whilst adiabatic storage concepts achieve efficiencies of up to 70 %.